1. Field of the Invention
The present invention relates to an optical information read/write apparatus.
2. Description of the Related Art
A hard optical disk is conventionally used for recording optical informations. The informations are written fixedly or unchangeably in advance in the disk. Instead of such a hard disk, an optical read/write disk in which informations can be written and read at any time is being developed.
Such a read/write disk has a larger record capacity than the hard disk, which enables to reduce the cost of the disk per bit. However, the read/write disk has a problem that the access time is long which makes the speed of the data transmission become slow.
In order to solve such a problem, a separation type optical pickup device has been developed. The pickup device is divided to two parts, i.e., a fixed unit and a movable unit. The movable unit is constituted to be light so that it can be moved at a high speed to shorten the access time.
There is another type of optical pickup device to solve the above mentioned problem. This device comprises two pickup units disposed opposite to each other with respect to the axis of the optical disk, one being used for writing informations in the disk while the other being used for reading informations from the disk. This type of optical pickup is called a boxer type pickup device. In accordance with such a double pickup structure, it becomes possible to read the informations for verify check while writing the informations, which enables to raise the speed of the data transmission.
The opticla information read/write apparatus having the above-mentioned double pickup structure is further described below.
The apparatus comprises a casing in which an optical disk is housed and set at the center thereof. The disk is driven to rotate in the casing by a drive means.
Two pickup units are disposed opposite to each other with respect to the rotary axis of the disk. One of the units is used for reading informations from the disk and composed of a fixed element and a movable element which is movable on a guide element for seeking motion. The other unit is used for writing informations on the disk and also composed of a fixed element and a movable element which is movable on a guide element for seeking motion.
Each of the fixed units is mounted on a stationary portion of the casing and has a light source and an optical sensor disposed therein.
Each of the movable units includes an objective lens and an actuator for driving the lens disposed therein.
In accordance with the above-mentioned double pickup structure of the read/write apparatus, since two pickup units have to be disposed in the casing at positions opposite to each other with respect to the axis of the disk, the size of the whole structure becomes large, which sometimes involves in a problem of layout of the apparatus.
One of the arrangements to reduce the size of the apparatus of the double pickup structure is to dispose the two pickup units on the same side with respect to the disk center instead of disposing them in the opposite sides each other with respect to the center. However, in such an arrangement, the guide members for guiding the movable element for seeking motion are disposed side by side in parallel to each other, which makes at least one of the guide members dislocated from the radial direction intersecting the axis of the disk. Therefore, the arrangement can not achieve a desirable seeking movement of the movable unit in which the unit is guided along the radial direction passing through the rotary axis of the disk.
Besides, the optical spot of the scanning laser beam converged on the disk is slightly oval due to the far field pattern of the laser diode of the light source. Therefore, in an inside track of the disk, the angle of the longitudinal direction of the spot with respect to the tracking direction becomes large compared to the angle in an outside track, which causes jitter of the reading signal and error of track positioning.